1. Field of the Invention
This invention relates to a system for utilizing audio wiring at drive-in theaters to transmit radio frequency signals to patrons' car radios with minimal electromagnetic wave leakage, and more particularly, to a system utilizing impedance matching networks and termination networks to enable the audio frequency wiring circuits to operate as transmission lines.
2. Description of Prior Art
In communicating the sound track portion of a motion picture to its patrons, drive-in theaters have utilized different types of systems. In one system audio from a movie projector is amplified by a high power audio frequency amplifier and then transmitted by means of underground wiring to many separate speakers distributed within the viewing area of the drive-in. This system requires that the number of speakers at least equal the number of automobiles within a drive-in theater, in order to deliver the sound track of the motion picture to patrons within each automobile. This type of audio distribution system has many shortcomings. First, each speaker must be of an outdoor all-weather type with the result that the audio quality usually is poor. Furthermore, it is relatively expensive to install and maintain the speakers even though they are constructed for outdoor use. Also, the speakers are subject to customer abuse, and maintenance and replacement costs are relatively high.
A second type of audio distribution system for a drive-in theater operates at radio frequencies and utilizes the audio from the sound track of the motion picture to modulate a radio frequency carrier having a frequency within the standard AM broadcasting band (540-1600 kilohertz). The radio frequency signal is then transmitted by wires to each patrons' automobile where it is coupled to the radio antenna. The radio frequency signal is then demodulated by the automobile radio and the motion picture audio sound is generated by the radio speaker. Since the audio quality of this radio frequency system is generally superior to that of the audio frequency system, it is desirable for drive-in theaters using an audio frequency system to convert to the radio frequency system. However, the conversion to a radio frequency system is relatively expensive because of the need to replace audio wiring with coaxial cables.
A partial solution to the expense of converting from an audio wiring system to a radio frequency signal system is disclosed in Herzfeld, U.S. Pat. No. 2,979,607. Herzfeld discloses a radio frequency system wherein the wiring formerly used for carrying the audio frequency signal is used to carry radio frequency signals from a centrally located radio frequency transmitter to each automobile radio antenna. One disadvantage of this arrangement, however, is that unless the output impedance of the radio frequency transmitter matches the impedance of the input end of the audio wiring, most of the radio frequency power will not reach the automobile radio antennas but rather will be reflected back into the transmitter. Another disadvantage of this arrangement is that unless the many output ends of the audio wiring are properly terminated, the audio wiring will act as a radiator instead of a transmission line. Since it is necessary to prevent interference with authorized transmissions having the same frequency as the drive-in system, it is accordingly desirable to utilize a system which limits radiation from the audio wiring.